Extrusion-type pellet mills and the process of producing pellet material using such devices are well known in the art. In pellet mills, a mixture of material to be pelleted, or “feed,” is typically fed to a die having a plurality of extrusion holes. Pellets are generally formed when the feed is compressed and molded between a pressure roll and an extrusion die.
During the extrusion process, generally one or more extrusion rolls travels over the compression side of the die and forces the material between the die and the rolls. This movement squeezes the material through extrusion holes in the die. As the material emerges from the discharge side of the die, the extrusions are severed to produce pellets. Other parts of the pellet mill may facilitate the continuous compression of feed between the pressure rolls and the die and the handling of the extruded pellets.
Each pellet mill is generally equipped with a die and roll assembly which often includes a plurality of pressure rolls, an extrusion die, and a mechanism for delivering feed material evenly along an inner surface of the extrusion die so that the feed can be compressed by the pressure rolls when they roll over the inner surface of the die. The inner surface of the die is also known as the compression surface or the extrusion surface. It is desirable to maximize production of pellets over a period of time. However, the need for frequent roll maintenance may limit pellet production efficiency.
FIG. 1A is a simplified diagram of a prior art pellet mill 100. The pellet mill 100 in FIG. 1 is an example of a common pellet extrusion mill. A die cover 111 is shown in an open position displaying a ring die 110 that is mounted upon a main shaft 120. The ring die 110 rotates around the main shaft 120. The ring die 110 may be powered, for example, by affixing the ring die 110 to the main shaft 120 and driving the main shaft 120 so the ring die 110 rotates in rigid rotation with the main shaft 120. Alternatively, the main shaft 120 may be fixed, and the ring die 110 may be driven by an external motor 112 via, for example, a belt 113, or an external friction roller (not shown) in contact with the exterior of the ring die 110. The inner and outer surfaces of the ring die 110 contain a plurality of extrusion holes 130. The feed material is fed into the ring die 110 by, for example, an auger 165, and forced into the extrusion holes 130 on the interior surface of the ring die 110, and emerges from the extrusion holes 130 on the exterior portion of the ring die 110. The extruded material may then be cut, for example, with a blade (not shown) to form pellets.
The feed in the pellet mill 100 is forced through the extrusion holes 130 by multiple rolls 140. Note that while three rolls 140 are depicted in FIG. 1A, prior art pellet mills may have one, two, three, four, or more rolls. Each roll 140 is freely rotating around a roll shaft 160, and each roll 140 is mounted on a carriage 150 (FIG. 2). The carriage 150 (FIG. 2) may be stationary, or may be driven to rotate around the main shaft 120. When feed is introduced to the interior of the ring die 110 by a feed path, for example, by the auger 165, attached to the die cover 111, the feed is driven toward the rolls 140. Note that in FIG. 1A the auger 165 is shown as separated from the rolls 140, for the purpose of clarity. When the die cover 111 is closed over the ring die 110, as shown in FIG. 1B, the auger 165 is positioned in the center of the die 110 so that the auger 165 may distribute feed material to the rolls 140.
In general, the rolls 140 do not come into direct contact with the inner surface of the ring die 110. Each roll 140 is separated from the ring die 110 by a pinch gap 170, as shown in FIG. 2. In order to provide optimum performance, it is desirable to monitor the size of the pinch gap 170 between the roll 140 and the die 110, and further to adjust the pinch gap 170 size as needed. For example, there may be variations in consistency of the feed over time, requiring either more or less pinch gap 170 pressure between the roll 140 and die 110 for optimum performance, and to compensate for wear on the roll 140 and die 110. However, since the rolls 140 are free rotating within the ring die 110, generally pinch gap 170 size, pressure and temperature are not monitored. Further, in multi-roll extrusion ring dies, the gap size must generally be adjusted on a per-roll basis, for example, by adjusting where the roll 140 is located upon the interior roll carriage 150. Such adjustments have heretofore required opening the ring die enclosure 111 (FIG. 1A) to access the rolls 140 and roll carriage 150. Therefore, the pinch gap 170 size in prior art ring dies 110 is generally static, such that it cannot be adjusted during normal operation, or in response to real time conditions.
The roll assemblies operate in a harsh environment. The rolls must be sealed to prevent the feed from entering the roll mechanisms, and to prevent lubricants or coolants from excessively leaking from the roll assemblies into the feed material. Since the rolls are generally located within a partially sealed die, they are generally difficult to access for maintenance and repair. The rolls are constantly subjected to high pressure in order to force the feed through the extrusion holes, and this pressure generates friction, heating the rolls and die as they rotate.
Since most rolls within a die extrusion ring are passive, they rely on the friction of the feed between the roll and the die to rotate the rolls. In some instances, particularly upon startup, the rolls may slip instead of rotating, causing vibration through the pellet mill and possibly causing additional wear to the components. However, since the rolls are typically mounted within the sealed extrusion die, it has heretofore been impractical to employ powered rolls, due to the difficulties of both driving the rolls and maintaining adequate pressure between the rolls and the die ring to force the feed through the extrusion holes. Some previous systems have mounted multiple rolls on a carriage within the extrusion die, and power the carriage so that the carriage rotates within the die, rotating the rolls around the interior surface of the die. However, in such systems the rolls themselves are still free rotating, and may therefore still experience slippage.
Roll bearings bear the pressure used to force the feed through the extrusion holes, and thus experience heat and stress, limiting lifespan of the roll. In general, larger bearings hold up better under higher pressure. Since the bearings have heretofore been located within physical confines of the roll itself, the maximum size of the bearing has been limited by the physical size of the roll. While it was possible to increase the size of the roll to facilitate larger bearings, the larger roll would be subjected to proportionally greater pressure, therefore generally negating the advantage sought from larger bearings. In order to facilitate the larger bearings, the size of the roller infrastructure has typically been reduced. This interdependency has necessitated a tradeoff between roller structural stability and bearing size.
The roll bearings typically require significant maintenance. Since the bearings are generally located within the rolls, which, in turn, are located within the ring die, servicing the roll bearings requires opening the die housing to access the rolls. This may lead to significant down time in the pellet manufacturing process. Furthermore, the complexity of rolls with integrated internal bearings may complicate the procedure for replacing and repairing the rolls. As a result, roll and bearing maintenance is time consuming, expensive, and causes an interruption of pellet production. In addition, frequent servicing of the rolls may increase safety risks to maintenance personnel, due to the high heat and pressure related with the die and rolls.
Therefore, there is an unmet need for an extrusion ring die having improved cooling and lubrication characteristics, that may be serviced less frequently than previous pellet mills, and that may be serviced without generally requiring frequent access to the interior region of the ring die. Further, there is a need for monitoring the pinch gap distance between the roll and die, as well as the pressure on the feed and the temperature of the roll, and to adjust the pinch gap size during system operation.